A conventional power supply of an electric machine such as a mobile (portable) telephone set in a car is connected through an ignition (key) switch to a car battery, so that an electric power is supplied to the telephone set, when the ignition switch is turned on. Therefore, the supply of an electric power is shut off to avoid unintentional consumption thereof, when the ignition switch is turned off. This structure is adopted, for the reason that electric power is consumed in the telephone set during not only in an occupied state for communication, but also in a waiting state for receiving a call. Consequently, the exhaustiveness of the car battery is avoided by turning the ignition switch off.
However, the conventional power supply of an electric machine in a car has a disadvantage in that an absentee (caretaking) recording of telephone message, an absentee receiving of facsimile information, etc. are not carried out, when an ignition key is pulled out and a driver leaves the car, so that the ignition switch is turned off to shut off the supply of an electric power to the telephone set.